1. Field of the Invention
The present invention relates to an optical wavelength stability control apparatus for stabilizing an optical wavelength output from a laser diode (hereinafter, LD). In particular, the present invention relates to an optical wavelength stability control apparatus suitable for an optical multiple wavelength transmission.
2. Description of the Related Art
Due to the development of an advanced information society, an optical communication system to which an optical signal is transmitted by using an optical fiber requires an enlarged transmission capacity. The optical multiple wavelength transmission is implemented to realize an increase in transmission capacity. A plurality of channels are transmitted through a common transmission path by assigning respective signals to different optical wavelengths. The precision stabilization of the optical wavelength within .+-.0.2 nm has long been required so that adjacent wavelengths do not interfere with each other.
FIG. 2 illustrates a conventional apparatus for optical wavelength stabilization. In general, it is known that a temperature fluctuation as well as a drive current fluctuation of a semiconductor laser cause a fluctuation of the optical transmitter. FIG. 2 illustrates an apparatus used for stabilizing an optical wavelength by keeping the temperature of a semiconductor laser 5 constant. A temperature monitor 10 detects the temperature of LD using a thermistor 9 and a reference voltage generator 3b outputs a reference temperature voltage which is a target value for controlling a temperature. An output voltage (Vth) of the temperature monitor 10 and an output voltage (Vref1) of the reference voltage generator 3b are compared at a comparator 8, and the difference between Vth and Vref1 is calculated. In a current controller 11, the stabilization of the optical wavelength is done by determining a drive current value of a thermoelectric cooler 12 so that an output value at the comparator 8 becomes zero. A semiconductor laser apparatus described in a Japanese laid-open patent No 57-186383 also employs the same method.
However, the electric power consumption (an input electric power to the semiconductor laser) required to obtain the identical optical power output gradually increases over time with the age of a semiconductor laser. Thus, the temperature at an active layer of the semiconductor laser rises and thereby causes an optical wavelength to fluctuate.
Japanese laid open patent 6-283797 describes a control method for keeping an optical power output and the temperature of the active layer constant. According to this method the temperature of a heat sink is controlled to negate a temperature rise of the active layer caused by an increase of the electric power consumption to gain an identical optical power with respect to an age related change of the semiconductor laser. Based upon this control, the temperature of the laser can be constantly controlled for a long period of time.
However, even if the temperature of the laser could be made constant, there is a problem that the optical wavelength of the laser changes in accordance with the fluctuation of the drive current when it is varied. In other words, as shown in FIG. 3, efficiency decreases with the age of the LD. To compensate for this deterioration, the LD drive current is controlled by an auto power control circuit (hereinafter, APC) so that the optical output of the LD becomes constant. Therefore, as shown in FIG. 4, the LD drive current value If (t) increases. The relation between optical wavelength and LD drive current is shown in FIG. 5.
Then, the fluctuation of the LD drive current causes fluctuation of the wavelength. A timing chart of an operation and a wavelength fluctuation in the conventional art is shown in FIGS. 6(a)-6(d).
When the LD drive current If (t) fluctuates with respect to an aging deterioration as shown in FIG. 6(a), a quantity of the wavelength fluctuation increases and the fluctuation cannot be compensated because a reference voltage (Vref1) is a fixed value as shown in FIG. 6(b).
The above-mentioned characteristics are explained using the following equations. A quantity of the wavelength drift (.DELTA..lambda.1) causing an increase/decrease of the LD drive current is given as equation 1. EQU .DELTA..lambda.1=.alpha..multidot.{If(tn)-If(t0)} (1)
where
.alpha.=Drive current-wavelength fluctuation conversion constant, PA1 If(t0)=Drive current value at initial time t0, and PA1 If(tn)=Drive current after passing time tn. PA1 G=Feedback loop gain, PA1 Vatc=Normalization portion output voltage value, PA1 .beta.=Temperature of the laser--Wavelength conversion constant, and PA1 .gamma.=Temperature in a circuit--Voltage conversion constant. PA1 Vth=Temperature monitor output (LD temperature), and PA1 Vref=Reference voltage generator output (initial set temperature). PA1 a LD drive current detector for detecting the LD drive current, PA1 a LD drive current increase/decrease normalization unit for outputting the increased or decreased LD drive current value being normalized, based upon the detected LD drive current value, PA1 a compensated reference voltage generator for generating the LD temperature control target value in response to an increase or a decrease of the LD drive current value being normalized, PA1 a temperature monitor circuit for detecting a temperature of the LD based on the thermistor, PA1 a comparator for detecting a difference between the detected LD temperature value and the LD temperature control target value, PA1 a current controller for determining a current value applied to the thermoelectric cooler so that a value detected by the comparator becomes zero, and PA1 a thermoelectric cooler for applying to the thermoelectric cooler a current value determined by the current controller. PA1 a LD, PA1 a current detector for detecting respectively the drive current driving the LD, and PA1 means for controlling respectively the temperature of the LD, PA1 wherein the means for controlling includes a temperature detection means for detecting respectively the temperature of the LD, a cooling means for cooling respectively the LD and a control means for setting respectively a control target temperature for the respective LD using a conversion coefficient predetermined for the respective LD in response to the variation of the detected drive current and controlling the cooling means to regulate the detected respectively the temperature of the LD to a set control target value.
On the other hand, a quantity of the wavelength drift (.DELTA..lambda.2) caused by a control loop error of a current controller is given as equation 2. EQU .DELTA..lambda.2=(1/G).multidot.Vatc.multidot..beta..multidot..gamma. (2),
where
Accordingly, a quantity of the wavelength drift (.DELTA..lambda.) in the optical wavelength stability control method of the conventional art is given as equation 3. EQU .DELTA..lambda.=.DELTA..lambda.1+.DELTA..lambda.2=.alpha..multidot.{If(tn)- If(t0)}+(1/G).multidot.Vatc.multidot..beta..multidot..gamma. (3)
Equation 4 is obtained from a feedback stability condition. EQU Vatc=G.multidot.(Vth-Vref1) (4)
where
When the equation 4 is substituted into the equation 3, .DELTA..lambda. is given as equation 5. EQU .DELTA..lambda.=.alpha..multidot.{If(tn)-If(t0)}+(Vth-Vref1).multidot..beta ..multidot..gamma. (5)
From the equation 5, it is confirmed that it is impossible to compensate the wavelength drift .alpha..multidot.{If(tn)-If(t0)} causing an increase/decrease of the LD drive current, even though the thermal detection voltage Vth and the reference voltage Vref 1 can be controlled.